1. Field of the Invention
The invention relates to a device and a method for determining the operating parameters of individual fuel cells or short stacks of fuel cells, preferably medium-temperature or high-temperature fuel cells. Thus, for instance, the operating parameters of solid oxide fuel cells (SOFC) or of molten carbonate fuel cells (MCFC) may be determined.
The invention also relates to a device and method for cooling hot process gases which arise during the operation of fuel cells or during the testing of components of fuel cells systems, such as reformers, mixing or conditioning systems or catalysers, at a fuel cell testing station.
The invention furthermore relates to a fuel cell stack made up of medium- or high-temperature fuel cells, which are provided with clamping elements acting on both ends of the fuel cell stack in order to compensate the internal operational pressure and/or seal the individual fuel cells.
2. The Prior Art
Devices and methods for the determination of individual cells or short stacks of cells serve to characterize or test fuel cells, for instance as regards their dependence on the temperature distribution over the cell surface.
The operational characteristics of individual cells and short stacks of, for instance, solid oxide fuel cells (SOFC) must be measured in the development process as well as in quality control. Up to now such tests and quality control measurements have usually been performed in a furnace kept at temperatures in the range from 300° C. to 1000° C., in order to guarantee a high and uniform ambient temperature.
Temperature distribution over the plane of the cell surface is influenced by various factors. In SOFCs there is primarily the orientation of flow from anode to cathode (cross-, co-, or counter-flow), the internal rate of the reforming reaction at the anode, and the flow volume and entry temperature of the cooling air. Besides, the design of the fields of flow will influence the temperature distribution.
The problems to be solved may be described as follows:                The data obtained in tests on the individual cell should permit direct inference of the characteristics of the whole stack of fuel cells. A furnace can simulate the unequal temperature distributions over the cell surface, as they occur in the stack before and behind the tested cell, only by a constant temperature.        A specific setting of temperature distributions in order to study the influence of different temperature distributions on the cell characteristics (electrical power output, service life, etc.) for example, is not possible.        Due to the low efficiency of heat transfer in the furnace between the heated furnace air and the fuel cell stack or the individual cells, heating performance is limited. Thus only slow heating-up is possible.        The gradients which arise when a SOFC consisting of many individual cells is heated, cannot be simulated realistically, i.e. impressed on the individual cell.        Parameters for simulation models have to be derived from experimental operating data which have been measured at an inhomogeneous temperature distribution not conducive to accurate measuring.        
It is an object of the present invention to propose improvements on devices and methods for measuring the operating parameters of individual cells or short stacks of medium- or high-temperature fuel cells as described above, such that the temperature situation prevailing in a fuel cell stack may be better simulated for development or quality control purposes.
In fuel cell testing stations (FCTS) the process gases have to be conditioned as regards thermodynamic characteristics (pressure, temperature, flow volume) and also concerning the composition of the gases. This may for instance be done with the help of a gas mixing station (=a combination of mass flow controllers) and electrical heaters or heat exchangers. The gas streams for anode and cathode are preheated up to temperatures of 800° C. before being fed to the fuel cell components (e.g. reformer or stack). Depending on the chemical or electrochemical reaction in the fuel cell components the temperature of the gas exiting the components may be higher or lower. In most cases, however, an additional cooling device for the process gases will be required before they can be passed to the exhaust hood of the testing station (danger of explosion!). In addition, the process gases in the exhaust vent must be suitably diluted to significantly below the explosion threshold.
Fuel cell stacks are usually subjected to an electrical load by means of an electronic device. The heat generated thereby must also be carried off. Air- or water-cooled devices for this purpose are marketed.
Finally, the chemical and electrochemical reactions in a fuel cell stack will generate heat, which also has to be carried off from this component by means of a cooling device.
Fuel cell testing stations thus require a connection to an external supply of cooling water. An external cooling water loop will significantly increase the cost of laboratory infrastructure, however.
From DE 199 13 795 C1 there is known a device with an internal combustion engine and a fuel cell system, in which the fuel cell assembly supplies electrical energy to the electrical units of the vehicle. By using some components, such as cooler, exhaust gas system and air filter, jointly for both the internal combustion engine and the fuel cell system, a reduction of the operating expense is achieved. Furthermore, the fuel cell system can be heated up by the exhaust gases generated by the internal combustion engine.
A further object of the invention will be to improve a device or a method for cooling hot process gases, which arise during the operation of fuel cells or the testing of components of fuel cell systems, such as reformers, mixing or conditioning systems or catalysers, in a testing station for fuel cells, in such a way that an external cooling water supply can be dispensed with.
In order to compensate the internal operating pressure and/or to seal the individual cells and/or to assure good electrical contact between intermediary plates/bipolar plates and electrodes, forces have to be exerted on the fuel cell stack. In known fuel cell designs these forces are applied via the fuel cell housing or via separate tensioning or clamping mechanisms.
At temperatures above 300° C. the material strength of these clamping mechanisms is significantly reduced, thus requiring relatively large masses to supply the mechanical forces. At temperatures above 600° C., which occur in the case of solid oxide fuel cells (SOFC) or molten carbonate fuel cells (MCFC), special metallic materials are required, leading to increased costs.
Thermal expansion of the fuel cell during start-up must be allowed for in the clamping mechanism by providing compensating elements adjacent to the fuel cell. In operation this will result in an inhomogeneous temperature distribution, since the clamping mechanism and the compensating elements act as cooling surfaces.
In conventional fuel cell stacks the heat capacity of the clamping mechanism will delay start-up and cause an inhomogeneous temperature distribution during start-up. In the case of frequent cold-starts heating up the additional masses will significantly increase fuel consumption.
In WO 03/028141 A2, for instance, a solid oxide fuel cell is described, which comprises a stack of individual cells clamped together by a clamping mechanism consisting of a base plate and a clamping plate. Between the clamping plate and the fuel cell stack there is placed a corrugated bellows made up of a number of bellows elements, which compensates the thermal expansion of the fuel cells during start-up. The corrugated bellows is made from heat-resistant metal alloy and is filled with a gas, for instance air at atmospheric pressure or an inert gas at higher pressure. Disadvantageously, during operation of the fuel cell an inhomogeneous temperature distribution will result since the base plate of the clamping mechanism and the metallic corrugated bellows will drain heat from the adjacent fuel cells and have differing thermal conductivities. A further disadvantage lies in the fact that start-up of the fuel cell or reaching of the optimum operating temperature is delayed due to the heat capacity of the clamping mechanism.
It is a third object of the invention to propose improvements for a fuel cell stack with a clamping mechanism as described, which will provide a more homogeneous temperature distribution in the start-up phase and permit the use of lighter, less expensive materials for the clamping mechanism.